The present invention concerns a display assembly including at least two superposed display devices and more particularly a timepiece including a display assembly of this type in which an analogue display device is combined with a digital display device.
An electronic watch including a case in which both an analogue display device and a digital display device are arranged is already known from French Patent FR 2 462 283. The analogue display device includes an hour hand and a minute hand which move above the dial in a conventional manner, while the digital display device includes a transparent liquid crystal cell arranged above the analogue display device which it completely covers. In this Patent, the digital display device allows the display of alphanumeric characters, for example the day of the week and the month in a dark colour on a light background. The liquid crystal cell forms the watch crystal. It is a twisted nematic liquid crystal including two polarisers arranged on either side of the cell, the dial which is situated under the cell being used as a reflector. With such a structure, the reflector and thus the watch dial, must reflect the incident light without modifying its polarisation in order to produce a brilliant high-contrast display. This display device thus limits the type of dial which it is possible to use to form the lower display device, in particular when the lower display device is a digital display device. In such case, only dials having non diffusing reflective properties, in other words metal reflectors, can be used.
Moreover, this type of cell generally displays data in a dark colour on a light background, which excludes the use of dark dials as reflectors. Further, the light background actually appears rather greyish because of the absorption of a non negligible quantity of light by the polarisers, which gives the display device unsatisfactory legibility and brightness.
Such a limit on the choice of dials represents a significant drawback for watch manufacturers who wish to be able to put watches including all types of dials on the market in order to adapt to changes in fashion.
Further, since the reflection of light occurs on the dial, and thus at a certain distance from the cell, typically 2 to 3 mm, the images and segments switched from the cell appear in projection on the dial, which leads to optical splitting of the displayed data. This is detrimental not only to the aesthetic appearance of the watch but also, of course, to the legibility of the displayed data.
An object of the present invention is to overcome the drawbacks of the aforementioned prior art by providing a display assembly including two superposed, respectively lower and upper, display devices, in which the brilliance of the data displayed by the upper display device and thus the legibility thereof, is not dependent or hardly dependent upon the nature (diffusing/reflective) of the dial of the lower display and its colour (light or dark).
Another object of the present invention is to provide a display assembly in which the upper display device is a liquid crystal device with an improved display contrast.
Another object of the present invention is to provide a timepiece fitted with such a display assembly having an improved aesthetic appearance.
The invention therefore concerns a display assembly including two superposed, respectively upper and lower display devices, the upper display device including a display cell arranged to be transparent in a first state to make the lower display device visible, and to display an item of data in a second state, control means supplying a control voltage to the cell to cause it to switch from the first state to the second state and vice versa, characterised in that said cell is a liquid crystal display cell of the diffusing or reflective type in the second state.
As a result of these features, the reflection of the incident light penetrating the cell no longer occurs on a dial placed behind the cell, as was the case in the prior art, but directly on the liquid crystal cell when the latter is switched into the second state. On the one hand this allows any projection interference of the switched zones on the dial to be removed, and thus the double display of such zones to the user to be prevented and, on the other hand, leaves watch manufacturers complete freedom in the choice of colours and the nature of the dial or lower display device. Such a display structure further increases the brilliance of the data displayed and, consequently, the legibility thereof, in particular when the lower display device is dark or includes a dark dial.
According to an advantageous feature of the invention, the liquid crystal cell is selected from the group formed by reverse PDLC cells, nematic gel or dynamic scattering cells, polymer stabilised chlolesteric texture cells (PSCT), electrolytic cells, and from twisted nematic cells provided with a polariser placed in front of the cell, having one quarter-wave plate and a cholesteric film placed successively behind the cell, and from twisted nematic cells provided with a first polariser placed in front of the cell and a reflective polariser which may or may not be associated with a second polariser placed behind the cell.
According to a first embodiment, the display assembly includes a cell of the twisted nematic type fitted with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film placed successively behind the cell, the upper display device or the lower display device further including a second quarter-wave plate placed behind the cholesteric film and the lower display device including a metal reflector placed behind said second quarter-wave plate.
The passage of light through the second quarter-wave plate allows the circular polarised light originating from the cholesteric film to be transformed into linear polarised light for which the metal reflector is more efficient. This structure allows the reflected light to be homogenised by the display assembly and the colour thereof to be made less dependent upon the wavelength of the light.
According to a second embodiment, the display assembly includes a cell of the twisted nematic type provided with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film having a first helical direction, placed in succession behind the cell, the lower display device including a cholesteric mirror having an opposite helical direction to that of said cholesteric film.
As a result of this structure, all the light passing through the cholesteric film, and representing half of the incident light, is reflected by the cholesteric mirror, which allows the brilliance of the display assembly to be improved. In the case of a wristwatch or suchlike, the cholesteric mirror can advantageously form the dial of the watch. Another advantage of this structure lies in the fact that one can choose cholesteric mirrors of different colours, which allows coloured display devices to be achieved.
According to a third embodiment, the optical valve includes a cell of the twisted nematic type provided with a polariser placed in front of the cell, and a quarter-wave plate associated with a cholesteric film placed in succession behind the cell, the lower display device further including a half-wave plate (xcex/2) and a second cholesteric film identical to the first.
As a result of this structure, one can use two same optical elements associated with the display cell, which represents an advantage from the practical point of view.
According to another embodiment of the invention, the display cell is in a transparent state in the absence of voltage applied thereto by the control means.
The data displayed by the lower display device is permanently visible without the upper display device consuming any energy. This is particularly advantageous within the scope of the application of this display device to a portable object such as a wristwatch.